On a BR Quad breaker, how does the breaker trip?

On the BR Quad breakers (ex. BQC breaker) the two outer breakers will be a common trip. The two inside breakers will be a common trip as well. These breaker should have a bar associated with them but that does not necessarily mean the breaker will trip. It is the mechanism in the breaker that enables the breaker to trip together at the same time.